What's it mean when a compressor is slugged? Well it's basically when liquid refrigerant or oil enters the compressor and either breaks the scroll or puts a hole through the compressor plates. LIQUID REFRIGERANT/OIL CANNOT BE COMPRESSED!!!  So, verify superheat, and possibly, install a crankcase heater depending on the application. 

 

"Liquid slugging is the term used to describe the passage of liquid refrigerant into the
scroll set or through the compressor suction and/or discharge valves. It is evidenced by a
loud metallic clatter inside the compressor, often accompanied by extreme vibration of the
compressor. If the compressor has a sight glass you will see an explosion of foam which is
liquid refrigerant boiling off. Slugging in refrigerant cooled compressors is from a severe
flooded start, which is a result from migration. Under extreme flooded start conditions, liquid
(oil and or refrigerant) can be drawn into the suction passages leading to compressor
cylinders or scroll set. When the compressor attempts to compress liquid, hydrostatic
pressure is developed, resulting in failure of oldham coupling, scroll set and reed or discus
valves."

Causes:
 - Refrigerant migration during 

   the off cycle
• Refrigerant migrates to the 
   coldest place as a vapor 
   and condenses
• Refrigerant settles below 
   oil
• On startup, oil is pulled into 
   the scroll set

 

• Prevent Flooded Start conditions which is a result of refrigerant vapour
migrating to the compressor crankcase oil or scroll during the offcycle.
 

• Operation and set points of the pump down control, and operation of the
crankcase heater should be checked and verified for correct operation.
 

Flooded Start Prevention:
 

• Install and use crankcase heaters anytime that ambient around the compressor
gets below 55F/12C because if the refrigerant wants to migrate to the coldest part
of the system and if the compressor has a crankcase heater that will reduce the
risk for flooded starts.
 

• Locate or relocate the compressor in ambient that will not cause migration
 

• Maintain a optimum refrigerant charge because there is something called
refrigerant to oil charge ratio. You may have all the above checking fine and you
still have flooded start issues, it could be the refrigerant to oil charge. Its a
refrigerant to oil charge limit for manufactures. The more refrigerant and the less
oil amount will result in more migration.